At least once a year. 15k is much to long an interval. Me, I’d do 6 months. An oil change is cheap insurance.
Don’t forget, the manufacturers don’t care what happens to your car once the warrantee period runs out. And they are under pressure from governments to reduce fluids use.
PS, the manuals usually list a time interval as well as a mileage interval. Such as “15k or 15 months whichever comes first”. What does your manual say?